CHROMOSOMAL  CHIMERAS   IN  CREPIS 


LILLIAN  HOLLINGSHEAD 


ty--^ 


University  of  California  Publications  in  Agricultural  Sciences 

Volume  2,  No.  12,  pp.  343-354,  plates  54,  55,  2  figures  in  text 

Issued  March  26,  1928 


University  of  California  Press 
Berkeley,  California 


Cambridge  University  Press 
London,  England 


CHROMOSOMAL  CHIMERAS  IN  CREPIS 


BY 

LILLIAN  HOLLINGSHEAD 


Gates  (1924)  stated  that  "it  is  unknown  at  the  present  time  how 
widespread  polyploidy  in  somatic  tissues  may  be."  Several  writers 
have  since  that  time  reported  cases  of  somatic  polyploidy  and  it  was 
thought  worth  while  to  put  on  record  instances  of  this  condition 
recently  discovered  in  the  Genetics  Laboratory  of  the  University  of 
Califoi'nia. 

In  the  course  of  an  examination  of  root  tips  of  various  Crepis 
species  and  species  hybrids,  two  plants  which  were  partly  tetraploid 
have  been  found. ^  These  are  not  the  first  cases  of  chromosomal  chi- 
meras reported  in  Crepis,  Lesley  (1925)  and  Nawaschin  (1926)  having 
previously  recorded  the  phenomenon.  Lesley 's  report  is  a  note  stating 
that  in  an  P^  between  C.  biennis  («  =  20)  and  C.  foetida  (w^5) 
a  few  neighboring  cells  were  found  having  about  twice  25  chromo- 
somes, whereas  most  of  the  cells  contained  the  expected  25.  Nawaschin 
reported  the  occurrence  of  a  tetraploid  area  in  the  form  of  a  narrow 
sector  in  a  diploid  root  of  C.  Bioscoridis  (w^4). 

The  first  of  the  two  cases  to  be  described  was  that  of  a  chimeral 
root  of  a  derivative  from  a  cross  between  Crepis  biennis  {n  =  20)  and 
C.  setosa  (^  =  4)  which  had  24  chromosomes  in  most  of  the  somatic 
cells.  In  this  root,  however,  a  large  number  of  cells  was  found  which 
obviously  had  many  more  than  the  noi*mal  24  chromosomes,  several 
approximated  48,  and  two  cells  gave  clear  accounts  of  48  chromosomes. 
The  normal  and  tetraploid  chromosome  complexes  are  shown  in 
figure  A.  By  an  examination  of  successive  sections  it  was  determined 
that  the  tetraploid  cells  were  confined  to  a  definite  region  which 
extended  from  at  least  vei-y  close  to  the  root  cap  to  a  point  where  no 
division  figures  could  be  found.  Apparently  the  longitudinal  outlines 
of  the  tetraploid  area  were  fairly  regular,  since  only  one  case  occurred 
in  which  diploid  and  tetraploid  cells  were  found  in  different  sections 
occupying  the  same  position  relative  to  the  circumference,  and  this 
was  on  the  line  of  demarcation  between  the  two  areas. 


1  Since  writing  the  above  a  root  of  C.  Hakelei  (/i^8)  containing  several 
neighboring  cells  with  about  twice  the  normal  chromosome  number  and  one  of 
C.  mothtana  (w^6)  with  one  tetraploid  cell  have  been  found. 


344  University  of  California  Publications  in  Agricultural  Sciences      [Vol.  2 

Figure  B  was  made  up  from  an  examination  of  the  whole  root  and 
shows  that  the  tetraploid  area  occupied  the  major  portion  of  the  root 
and  that  its  cross-section  was  very  irregular  in  shape.  The  dotted 
lines  indicate  the  portions  of  the  boundary  between  the  2n  and  4« 
areas  which  could  not  be  accurately  determined.  It  is  uncertain 
whether  the  tetraploid  area  extended  into  the  central  cylinder  or  not. 
The  tetraploid  area  at  a  is  two  cell  layers  deep,  at  h  it  is  only  one,  but 
opposite  a  at  c  it  occupies  most  or  all  of  the  cortex.  While  there  is 
considerable  variation  in  cell  size  \vithin  both  areas  the  average  size 
of  the  tetraploid  cells  is  larger  than  that  of  the  diploid. 


^8S 


>» 

^•^ 


Fig.  A.  1,  Diploid  chromosome  complex  of  biennis-setosa  liybrid  derivative 
(2n  =  24).  2,  Tetraploid  complex  (2w^48)  from  chimeral  root  of  the  same 
plant. 

The  shape  of  the  tetraploid  area  is  of  some  interest  from  the  point 
of  view  of  development  as  presumably  the  doubling  of  chromosomes 
took  place  in  one  of  the  initial  cells  from  which  the  root  developed. 
The  area  does  not  show  the  comparative  regularity  in  shape  exhibited 
by  Nawaschin's  tetraploid  sector.  On  the  other  hand,  tetraploidy  here 
is  confined  to  one  definite  area,  which  was  not  the  case  with  Lesley's 
(1925)  tomato  chimeras,  where  isolated  areas  of  tetraploid  cells  were 
observed.  The  condition  is  similar  to  that  which  Langlet  (1927) 
found  in  two  roots  of  Thalictrum  but  differs  from  that  reported  by 
Winge  (1927)  in  Tragopogon  hybrids  where  the  tetraploid  parts  of 
two  roots  by  reason  of  their  larger  cells  rendered  the  cross-sections 
of  the  roots  eccentric.  The  plant  bearing  this  chimeral  root  was  of 
normal  appearance  in  the  rosette  stage  but  unfortunately  died  before 
flowering. 

The  second  case  showing  a  chimeral  condition  was  a  plant  of 
Crepis  Burcniana  (?^  =  4).  Of  the  thirty  roots  of  this  plant  which 
were  examined  two  were  tetraploid,  having  16  chromosomes  in  all  the 


1928] 


Hollingsliead :  Chromosomal  Chimeras  in  Crcpis 


345 


plates  observed.  Plate  54,  figures  a  and  b,  shows  the  chromosome  com- 
plexes with  surrounding  areas  from  the  outer  cortex  in  comparable 
regions  of  the  diploid  and  tetraploid  roots.  Each  of  the  chromosomes 
of  the  diploid  complex  could  be  recognized  in  the  tetraploid  and  the 
longest  one  of  the  set  could  be  identified  four  times  in  the  best  tetra- 
ploid plate.  Undoubtedly  there  has  been  a  doubling  of  the  diploid 
set.  An  examination  of  plate  54  shows  that  the  average  size  of  cells 
and  nuclei  is  larger  in  the  tetraploid  root. 


Fig.  B.  A  cross-section  of  root  of  hiennis-setosa  hybrid  derivative  showing 
the  extent  of  the  tetraploid  area.  The  outer  ring  is  the  extension  of  the  root  cap 
and  contains  no  dividing  cells. 

The  occurrence  of  two  tetraploid  roots  in  the  same  plant  may  be 
taken  as  evidence  that  part  of  the  central  cylinder  from  which 
branches  arise  had  become  tetraploid.  It  was  thought  possible  that 
the  plant  above  ground  might  have  been  affected  similarly,  and  if  so, 
the  pollen  produced  on  a  tetraploid  branch  would  be  larger  than  that 
on  a  diploid.  Examination  of  pollen  fi*om  various  branches,  however, 
showed  no  noticeable  size  differences,  so  it  was  concluded  that  tetra- 
ploidy  was  probably  confined  to  the  roots. 

In  this  same  plant  a  root  was  examined  which  contained  a  number 
of  large  cells.  Most  of  them  Avere  clearly  multinucleate,  from  two  to 
four  nuclei  having  been  counted  in  single  cells  (pi.  55).  Plate  55  h 
shows  one  of  the  smallest  of  these  with  2  nuclei,  and  2  nuclei  may  be 
observed  in  one  of  the  cells  in  a.  In  the  larger  cells  nuclei  were  to  be 
found  in  successive  sections.     These  cells  were  scattered  throughout 


346  University  of  California  Puhlicaticns  in  Agricultural  Sciences      [Vol.  2 

the  cortex,  mostly  near  the  i^eriphery,  and  in  one  region  a  group  of 
them  seems  to  be  responsible  for  the  somewhat  misshapen  appearance 
of  the  root  in  cross-section  (pi.  55  a,c).  The  cells  vary  in  size,  some- 
times extending  through  three  or  four  sections  7/x  thick.  They  are 
more  or  less  vacuolated,  depending  on  their  size,  but  even  the  smallest 
ones  could  be  distinguished  almost  at  a  glance  by  their  less  densely  , 
staining  cytoplasm. 

The  normal  chromosome  complex  of  8  was  to  be  seen  in  several 
plates  of  normal  cells.  In  one  very  large  vacuolated  cell  a  large 
number  of  chromosomes  was  observed,  apparently  in  metaphase 
(pi.  55  c?).  Note  the  V-shaped  arrangement  of  the  metaphase  plate. 
The  chromosomes  could  be  seen  in  3  successive  sections  of  7/i,  thickness. 

Nawaschin  (1926)  has  reported  a  giant  cell  with  over  500  chi'omo- 
somes  in  a  root  tip  of  Crcpis  tectorum  and  takes  up  favorably  the 
theory  that  it  has  arisen  by  successive  chromosome  divisions  without 
accompanying  cell  divisions.  The  cell  he  shows  is  not  greatly  unlike 
•  some  of  tliose  seen  in  this  material.  Here,  however,  there  seems  to  be 
some  evidence  that  the  large  cells  are  the  result  of  fusion  of  several 
smaller  ones.  The  V-shape  of  the  one  plate  observed  in  a  giant  cell 
indicates  that  it  may  be  a  combination  of  two  plates  and  that  a  nuclear 
fusion  has  taken  place.  Plate  55  c  gives  the  impression  that  two  large 
cells  are  in  the  process  of  fusion  and  indeed  the  cell  wall  has  practic- 
ally disappeared.  It  seems  likely  that  such  cell  fusion  might  be  asso- 
ciated with  an  abnormal  or  pathological  condition  of  the  root,  further 
evidence  of  which  was  to  be  seen  in  small  black  areas  probably  repre- 
senting degenerated  cells  (pi.  55  b). 

The  origin  of  tetraploidy  in  diploid  tissue  has  been  discussed  by 
various  investigators.  In  some  cases  it  has  been  associated  with  specific 
outside  influences.  The  effect  of  narcotics  in  inducing  tetraploidy  has 
been  investigated  by  Nemec  (1903)  and  Sakamura  (1920).  Blakeslee 
and  Belling  (1924)  found  tetraploid  shoots  in  Datura  plants  subjected 
to  cold.  Lesley  (1926)  found  tetraploid  areas  in  tomato  plants 
affected  by  mosaic  and  thought  it  might  be  jiossible  that  local  changes 
due  to  this  disease  might  affect  mitotic  processes.  Cases  of  polyploid 
cells  have  been  attributed  to  abnormal  processes  related  to  degenera- 
tion as  in  the  investing  cells  of  the  ovaries  of  Anasa  tristis  (Wilson, 
1906).  The  doubling  of  chromosome  numbers  in  Winkler's  (1916) 
Avell-kno^Mi  chimeras  has  been  attributed  to  the  effect  of  wounding. 
Jorgensen  and  Crane  (1927)  have  recently  secured  tetraploidy  in 
Solaitum  by  the  use  of  Winkler's  method.     Winge  (1927)  finds  that 


3928]  Hnllingshead :  Chromosomal  Chimeras  in  Crepis  347 

most  of  the  cells  of  the  "crown  galls"  on  sugar  beets  which  can  be 
induced  by  inoculation  with  Bacterium  tumefaciens  have  the  tetra- 
ploid  chromosome  number. 

Nawaschin  did  not  venture  any  suggestion  as  to  causal  agencies  in 
connection  with  his  tetraploid  sector  in  Crepis  Dioscoridis.  Lesley 
believed  that  it  was  unlikely  that  cold  played  any  part  as  a  causal 
factor  in  tomato  chimeras,  as  only  the  roots  seemed  to  be  affected.  It 
has  been  suggested  by  Mr.  C.  W.  Haney  that  watering  greenhouse 
plants  with  cold  water  would  pro\'ide  the  necessary  conditions  if  sud- 
den lowering  of  temperature  has  anything  to  do  with  the  production 
of  tetraploid  root  cells.  The  plants  described  here  were  entirely 
normal  as  far  as  could  be  observed  and  tetraploidy  could  not  be 
ascribed  to  any  special  factor  in  the  environment. 

Winkler  had  suggested  that  certain  tissues  may  regularly  become 
polyploid  and  Breslawetz  (1926)  has  reported  tetraploidy  as  the 
universal  condition  in  the  dermatogen  of  the  root  tips  of  Cannabis 
sativa.-  De  Litardiere  (1923)  found  tetraploid  and  octoploid  cells 
in  the  deraiatogen  of  Spinacia  oleraeea.  In  both  these  cases  it  would 
seem  that  the  transforming  of  diploid  into  tetraploid  cells  must  have 
oceui-red  many  times  in  the  same  root. 

The  possibility  of  fragmentation  giving  rise  to  these  increased 
numbers  is  easily  excluded  in  most  cases.  The  two  most  favorably 
received  theories  to  account  for  doubling  are:  (1)  the  fusion  of  nuclei 
from  two  cells;  (2)  the  division  of  the  chromosome  complex  without 
cytoplasmic  division.  Breslawetz  has  brought  forward  evidence  that 
nuclear  fusion  gave  rise  to  the  tetraploid  cells  which  made  up  the 
dermatogen  of  the  roots  in  Cannabis  sativa.  As  no  diploid  complexes 
were  to  be  seen  in  that  region  of  the  root  one  would  conclude  that 
fusion  of  diploid  to  form  tetraploid  nuclei  had  taken  place  before  any 
normal  diploid  divisions  occurred,  or  at  least  at  an  early  stage  in  the 
development  of  the  root.  If  so,  one  wonders  why  evidences  of  nuclear 
fusion  were  still  to  be  found  in  well  developed  roots.  On  the  other 
hand,  the  paired  condition  of  the  chromosomes  in  some  of  the  tetra- 
ploid cells  in  Spinacia  oleraeea  convinced  de  Litardiere  that  these 
cells  had  just  completed  a  chromosome  division  without  separation  of 
the  resulting  daughter  chromosomes. 

The  occurrence  of  multinucleate  cells  in  a  root  of  the  Crepis 
Bureniana  plant  which  was  partly  tetraploid  has  been  noted  above. 
The  significance  of  this  phenomenon  in  the  origin  of  the  tetraploid 


2  De  Litardiere    (1924)    found  rare  diploid  cells  in  the  peribleni  and   in  one 
ease  a  tetraploid  cell  in  the  plerome  of  roots  of  this  species. 


348  University  of  California  Publications  in  A gricttltwal  Sciences      [Vol.  2 

roots  is  questionable.  The  presence  of  several  nuclei  in  one  cell  and 
the  abnormal  appearance  of  these  large  cells  would  incline  one  to 
belittle  the  possibility  that  tetraploidy  here  had  originated  by  cell  and 
nuclear  fusion.  However,  it  has  been  pointed  out  that  the  one  chromo- 
some complex  observed  in  a  giant  cell  indicated  that  nuclear  fusion 
had  taken  place.  We  cannot,  therefore,  dismiss  the  possibility  that  a 
fusion  of  nuclei  from  two  cells  gave  rise  to  a  cell  which  was  tetraploid 
and  thence  to  tetraploid  roots.  It  seems  more  likely,  however,  that  the 
occurrence  of  tetraploidy  and  of  a  root  with  giant  multinucleate  cells 
in  the  same  plant  was  merely  a  coincidence. 

Whatever  the  method  of  origin,  the  frequent  occurrence  of  tetra- 
ploidy in  somatic  tissues  throws  some  light  on  two  much  discussed 
questions.  First,  there  is  that  of  the  mode  of  origin  of  diploid  gametes. 
Rosenberg  (1926-27),  Karpechenko  (1927),  and  others  have  described 
processes  in  the  reduction  divisions  of  apogamous  species  and  inter- 
specific hybrids  bj'  which  diploid  gametes  are  formed.  The  increasing 
frequency  with  which  tetraploidy  has  been  recorded  in  root  tips  makes 
it  seem  likely  that  it  would  be  found  in  other  tissues  were  they 
examined  as  consistently.  Its  occurrence  in  the  cells  of  the  germinal 
line  would  lead  to  the  formation  of  gametes  with  twice  the  normal 
chromosome  number.  This  lias  been  shown  to  occur  in  Datura  where 
tetraploid  shoots  were  found.  Presumably  a  smaller  area  might  be 
affected  and  only  a  portion  of  the  gametes  formed  on  one  shoot  might 
be  diploid. 

In  the  second  place,  the  frequent  occurrence  of  somatic  tetraploidy 
has  a  bearing  on  the  origin  of  tetraploids  and  tetraploid  hybrids. 
Prirmda  kewensis  arose  as  a  bud  sport  probably  from  an  Fi  hybrid  of 
P.  verticillata  and  P.  florihunda.  It  has  the  sum  of  the  diploid  chro- 
mosome numbers  of  the  parent  species  and  Clausen  and  Goodspeed 

(1925)  have  suggested  that  it  is  a  true  tetraploid  hybrid,  the  bud  sport 
having  arisen  by  a  doubling  of  somatic  chromosomes.  A  similar 
explanation  with  the  doubling  occurring  immediately  subsequent  to 
fertilization  was  suggested  by  these  investigators  to  explain  the  origin 
of  a  tetraploid  hybrid  between  Nicotiana  tabacum  and  N.  glutinosa. 
Rosenberg  (1926)  has  recently  proposed  an  explanation  for  the  origin 
of  the  tetraploid  AcgUops — Tritic^im  hybrid  of  Tschermak  and  Bleier 

(1926)  which  depends  on  the  chance  meeting  of  diploid  gametes 
formed  by  a  " semi-heterotypic "  division.  In  the  light  of  the  fore- 
going facts  it  seems  much  simpler  to  suppose  that  a  doubling  of  the 
chromosomes  took  place  in  the  fertilized  egg,  or  in  some  cell  of  the 


1928]  Hollingshead :  Chromosomal  Chimeras  in  Crepis  349 

young  embryo  which  gave  rise  to  the  growing  point  of  tlie  stem. 
Nawaschin  was  led  to  favor  this  theory  of  the  origin  of  tetraploids  by 
the  frequency  of  4j(  plants  in  Crepis  tecforum.  He  calculated  the  fre- 
quency of  diploid  gametes  from  the  number  of  triploid  plants  obtained 
in  over  4,000  individuals,  and  on  this  basis  determined  the  number  of 
tetraploids  which  should  occur  by  chance  meeting  of  those  gametes. 
He  found  the  expected  number  of  tetraploids  to  be  much  less  than  that 
actually  occurring.  He  concluded,  therefore,  that  tetraploids  arose 
through  the  doubling  of  chromosomes  in  the  fertilized  egg  cells. 

In  view  of  the  increasing  number  of  cases  in  which  tetraploidy  has 
arisen  in  normal  diploid  tissue,  one  is  justified  in  concluding  that  it 
may  play  a  part  in  the  origin  of  polyploid  species  and  interspecific 
hybrids. 

I  acknowledge  with  pleasure  my  indebtedness  to  Dr.  J.  L.  Collins 
and  Professor  E.  Bi  Babcock  for  the  material  used  in  this  study. 


SUMMARY 

Tetraploidy  was  observed  in  the  roots  of  two  different  plants.  One, 
a  C.  biennis  x  C.  setosa  hybrid  derivative,  had  one  root  partly  tetra- 
ploid.  The  other,  a  plant  of  C.  Bureniana,  had  two  roots  wholly 
tetraploid. 

No  external  factors  could  be  associated  with  the  tetraploidy. 

Giant  multinucleate  vacuolated  cells  occurred  in  another  root  of 
the  same  C.  Bureniana  plant.  Evidence  of  cell  and  nuclear  fusion  was 
observed.  It  is  doubtful  whether  this  phenomenon  has  any  significance 
in  relation  to  the  origin  of  the  tetrajjloid  roots. 

Tetraploidy  arising  in  somatic  tissue  probably  plays  a  part  in  the 
origin  of  polyploid  species  and  intei-specific  hybrids. 


350  University  of  California  Puhlications  in  Agricultural  Sciences      [Vol.2 


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1928]  nollingsliead:  Cliromosomal  Chimeras  in  Crrpis  351 

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TSCHEBMAK,   E.,  UND  BLEIEH,   H. 

1926.  Tiber  fruehtbare  Aegilops  Weizenbastarde.     (Beispiele  fiir  die  Eutste- 

hung  neuer  Arten  durch  Bastardierung.)     Berichte  der  deut.  Bot. 
Gesell.,  vol.  44,  pp.  110-32. 

Wilson,  E.  B. 

1906.     Studies  on  chromosomes  III.     Jour.  Exper.  Zool.,  vol.  3,  pp.  1-40. 

WlNGE,  O. 

1927.  Zytologisfhe    Untersuehungen    iiber    die    Natur    maligner    Tumoren. 

I.    "Crown    gall"    der    Zuckerriibe.      Zeitsclir.    f.    Zellf.    u.    Mikr. 
Anat.,  vol.  6,  pp.  397-423. 

Winkler,  H. 

1916.  tJber  die  experimentelle  Erzeugung  von  Pflanzen  mit  abweichenden 
Chromosomenzahlen.     Zeitsclir.  f.  Bot.,  vol.  8,  pp.  417-.531. 


PLATE  54 
Comparable  areas  of  a,  diploid,  b,  tetrnploid  roots  of  a  Crepis  Burenkina  plant. 


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UNIV      CALIF      PUBL     AGR.    SCI    .    VOL      2 


HOLLINGSHEAD I     PLATE    54 


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PLATE  55 

Photomicrographs  of  root  of  the  Crepis  Burcniana  plant  showing  giant  multi- 
nucleate cells. 

a.  A  cross-section  showing  a  group  of  giant  cells. 
h.  One  of  the  smaller  giant  cells  with  two  nuclei. 

c.  Two  giant  cells  apparently  fusing. 

d.  A  large  cell  containing  a  V-shaped  metaphase  plate  with  many  chromosomes. 


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UNIV.    CALIF.    PUBL.    AGR.    SCI    .    VOL      2 


HOLLINGSHEAD I     PLATE     55 


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